1. Field of the Invention
In general, the present invention relates to an optical visualizing apparatus attached to the head of equipment such as an HMD (head mounted display) apparatus for viewing an image in front of a face. In particular, the present invention relates to an optical visualizing apparatus wherein a liquid crystal employed therein has the contrast of the visual-field angle thereof with an improved characteristic.
2. Description of the Related Art
FIG. 1 is a diagram showing an optical visualizing apparatus 50 used as an HMD (head mounted display) apparatus for viewing an image displayed by a liquid crystal in front of the eyes of an image viewer. As shown in the figure, the optical visualizing apparatus 50 comprises a head mounting unit 51 for supporting a display unit 58 in front of the eyes of an image viewer, the display unit 58 mounted in front of the eyes of an image viewer, standing face to face with the eyes of an image viewer, for projecting an image on both eyes of an image viewer and a linking unit 60 for linking the display unit 58 to the head mounting unit 51 and holding the display unit 58 in front of the eyes of an image viewer.
The head mounting unit 51 comprises a head supporting member 53 for supporting a forehead pad 52 brought into contact with the forehead, a pair of side cabinets 55 linked with the head supporting member 53 through hinges 54 at the ends of the head supporting member 53, a head-circumference adjustable belt 56 adjustably connected to ends of the side cabinets 55 and a hair band 57 adjustably supported by the side cabinets 55 and provided on the head. Linked to one end of the linking unit 60, the head mounting unit 51 has a curved shape approximately resembling the circumference of the head.
The display unit 58 comprises a cabinet with a shape approximately resembling goggles serving as a cover veiling the front face and a display body 59 protected by this cover.
FIG. 2 is a diagram showing the display body 59. As shown in the figure, the display body 59 comprises a cabinet 61, a liquid-crystal display element 62 for displaying an image inside the cabinet 61, a light-source unit 63 for supplying light to the liquid-crystal display element 62, a half mirror 64 capable of changing the optical path of an image from the liquid-crystal display element 62 and passing on the optical path and a concave mirror 65 for observing and enlarging the virtual image of a light beam with the optical path thereof changed by the half mirror 64.
FIG. 6 is a diagram showing the liquid-crystal display element 62. As shown in the figure, the liquid-crystal display element 62 comprises a liquid-crystal board 66 with a square shape for generating an image, a first polarizing board 68 provided above the liquid-crystal board 66 and a second polarizing board 69 placed beneath the liquid-crystal board 66. The liquid-crystal board 66 is thus sandwiched by the first and second polarizing boards 68 and 69. The first polarizing board 68 is used for polarizing a light in the longitudinal direction (or the vertical direction) with respect to the visual-field angle .theta. of the eyeball 67 of an image viewer. On the other hand, the second polarizing board 69 is used for polarizing the light in the transverse direction (or the horizontal direction) with respect to the visual-field angle .theta. of the eyeball 67 of an image viewer.
FIGS. 7A to 7C are explanatory diagrams showing the liquid-crystal board 66 as well as the first and second polarizing boards 68 and 69. As shown in FIG. 7B, the liquid-crystal board 66 has a visual-field angle of about 90 degrees in twist orientations with respect to the vertical and horizontal directions as seen from the front thereof. As shown in FIG. 7A, the first polarizing board 68 has a vertical polarizing direction as seen from the front thereof. As shown in FIG. 7C, the second polarizing board 69 has a horizontal polarizing direction as seen from the front thereof.
By placing the display body 59 equipped with the liquid-crystal display element 62 with such a configuration at a location close to both eyes 67 of an image viewer, the contrast characteristics of an image displayed from the liquid-crystal display element 62 are improved in the directions of the visual-field angle .theta. of the eyeball 67 of an image viewer. To be more specific, the contrast characteristic in the vertical direction with respect to the liquid-crystal board 66 is improved by the first polarizing board 68. On the other hand, the contrast characteristic in the horizontal direction is improved by the second polarizing board 69. As a result, an image displayed from the liquid-crystal board 66 has improved contrast characteristics in the vertical and horizontal directions. That is to say, the so-called optical transmittance is improved in both the vertical and horizontal directions.
However,a liquid-crystal display element that improves the characteristics of the contrast in the vertical and horizontal directions with respect to the directions of the visual-field angle of the liquid-crystal display element provided by the conventional technology described above has a poor contrast characteristic of a visual-field angle from a slanting direction. In order to correct the poor contrast characteristic, the amplitude is increased. When the amplitude is increased, however, part of the screen is inadvertently inverted, giving rise to a problem that a sharp contrast can not be obtained and it is thus impossible to improve the quality of the screen.
In order to solve this problem, the contrast in the horizontal direction with respect to the direction of the visual-field angle is changed. When the contrast is changed in this way, however, the contrast of an image on the right or left side is inadvertently changed in dependence upon the width of the eye, making the eyes of an image viewer become tired easily.